Convert rustc_serialize integration tests to unit tests.

Because (a) the vast majority of compiler tests are unit tests, and (b)
this works better with `unused_crate_dependencies`.

compiler/rustc_serialize/src/leb128.rs

@@ -155,3 +155,6 @@ impl_read_signed_leb128!(read_i32_leb128, i32);
 impl_read_signed_leb128!(read_i64_leb128, i64);
 impl_read_signed_leb128!(read_i128_leb128, i128);
 impl_read_signed_leb128!(read_isize_leb128, isize);
+
+#[cfg(test)]
+mod tests;

compiler/rustc_serialize/src/leb128/tests.rs

@@ -1,9 +1,6 @@
-// FIXME
+use super::*;
-#![allow(unused_crate_dependencies)]
+use crate::Decoder;
-
+use crate::opaque::{MAGIC_END_BYTES, MemDecoder};
-use rustc_serialize::Decoder;
-use rustc_serialize::leb128::*;
-use rustc_serialize::opaque::{MAGIC_END_BYTES, MemDecoder};
 
 macro_rules! impl_test_unsigned_leb128 {
     ($test_name:ident, $write_fn_name:ident, $read_fn_name:ident, $int_ty:ident) => {

compiler/rustc_serialize/src/lib.rs

@@ -3,8 +3,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::internal)]
-// FIXME
-#![allow(unused_crate_dependencies)]
 #![cfg_attr(test, feature(test))]
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
@@ -18,6 +16,10 @@
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end
 
+// Allows macros to refer to this crate as `::rustc_serialize`.
+#[cfg(test)]
+extern crate self as rustc_serialize;
+
 pub use self::serialize::{Decodable, Decoder, Encodable, Encoder};
 
 mod serialize;

compiler/rustc_serialize/src/opaque.rs

@@ -451,3 +451,6 @@ impl<'a> Decodable<MemDecoder<'a>> for IntEncodedWithFixedSize {
         IntEncodedWithFixedSize(u64::from_le_bytes(bytes))
     }
 }
+
+#[cfg(test)]
+mod tests;

compiler/rustc_serialize/src/opaque/tests.rs

@@ -1,13 +1,10 @@
-#![allow(rustc::internal)]
-// FIXME
-#![allow(unused_crate_dependencies)]
-
 use std::fmt::Debug;
 use std::fs;
 
 use rustc_macros::{Decodable_NoContext, Encodable_NoContext};
-use rustc_serialize::opaque::{FileEncoder, MemDecoder};
+
-use rustc_serialize::{Decodable, Encodable};
+use crate::opaque::{FileEncoder, MemDecoder};
+use crate::{Decodable, Encodable};
 
 #[derive(PartialEq, Clone, Debug, Encodable_NoContext, Decodable_NoContext)]
 struct Struct {